1. Field of the Invention
The present invention relates to a luminescent device, an optical coherence tomographic imaging apparatus provided with the luminescent device and a control method of the luminescent device.
2. Description of the Related Art
In recent years, attention has been attracted to a super luminescent diode (hereinafter abbreviated as SLD) that is a luminescent device.
The SLD mixedly uses both stimulated and amplified light and spontaneously emitted light unlike a semiconductor laser by which light with high output and very narrow spectral width is oscillated with a low injection current by using stimulated amplification and further resonating the light and unlike an LED which utilizes spontaneously emitted light and is wide in radiation angle.
A feature of this SLD resides in that the output is high compared with the LED and a wide spectrum half width compared with the semiconductor laser is obtained by constructing it so as not to undergo resonation even in a highly current-injected state. The SLD is widely applied to many uses such as a spectroscope, a length measuring machine, a refractive index distribution measuring device, a tomographic imaging apparatus and a light source for excitation by making good use of such a feature.
When such an SLD is used in the above-described application devices and the optical output and spectral form thereof are controlled, a method in which SLD light to be used is split and detected and a method in which light from an exit face opposite to a face to utilize SLD light used is detected and controlled are considered.
In the present specification, the light which does not undergo resonation even in a highly current-injected state and is stimulated and amplified is called SLD light.
Even in light emitted from the same active layer region, a spontaneously emitted light component and an SLD light component are present. In the present specification, these are separately described.
Specific examples of a position of a detector regarding control of SLD light are illustrated in FIGS. 8A and 8B. FIG. 8A illustrates a construction that SLD light 806 emitted from an SLD device 805 whose upper electrode is a single electrode structure is branched by means of a branching mirror 802. The SLD light 806 is divided into SLD light 803 to be used and light 804 entering in a detector 811, and a current injection amount to the upper electrode is controlled on the basis of a signal detected in the detector 811, whereby the optical output and spectral form of the SLD light are adjusted.
In FIG. 8B, SLD light 807 emitted from the side opposing an exit side of the SLD light 806 is detected in a detector 812, and a current injection amount to the upper electrode is controlled on the basis of a signal detected in the detector 812, whereby the optical output and spectral form of the SLD light are adjusted.
On the other hand, as described in A. T. Semenov, V. R. Shidlovski, D. A. Jakson, R. willsch and W. Ecke, “Spectral control in multisection AlGaAs SQW super luminescent diodes at 800 nm” ELECTRONICS LETTERS, Vol. 32, No. 3, p. 255 (1996) (Non Patent Literature 1), an attempt to use plural upper electrodes and independently control current injection amounts to the respective electrodes is made for realizing higher output and wider spectrum half width at the same time.
Here, the structure described in Non Patent Literature 1 is described with reference to FIG. 9A. An SLD device is composed of a ridge waveguide structure and an AlGaAs heterostructure having an 8 nm thick quantum well active layer (not illustrated).
In addition, the device has three upper electrodes 901, 902 and 902 each having a length of 500 μm. Among these electrodes, an upper electrode close to the side of an exit face of SLD light 906 is the first electrode 901. An electrode located at the center is the second electrode 902, and an electrode arranged on the side of another exit face 905 is the third electrode 903. In this Literature, current is applied to only the first electrode 901 and the second electrode 902, and the third electrode 903 is used as a light absorbing region without applying current (in other words, the end face 905 neither exits light not reflects light). FIG. 9B shows optical outputs and spectrum half widths of the SLD light 906 when current is injected into the respective upper electrodes. It is understood that the values of the optical outputs and spectrum half widths are greatly varied according to the amount of the current injected into the respective upper electrodes 901 and 902.
In addition, in Non Patent Literature 1, spectral forms under conditions (i) to (iv) are illustrated in FIG. 2, it is disclosed that the spectral form is also greatly varied according to the amount of the current injected into the respective electrodes.
However, the prior art structure described in Non Patent Literature 1 involves the following problems.
That is to say, in the case of the SLD having the plural upper electrode structure described in Non Patent Literature 1, it is difficult to take the same method as in a single electrode structure for respectively determining optimum amounts of the current injected into the plural upper electrodes to control both optical output and spectral form.
For example, when a case where a method of branching SLD light to be used and using a part thereof for monitoring (corresponding to FIG. 8A) is applied to the plural electrode structure is supposed, the following problem is caused.
Since the spectral form greatly vary due to a difference between the amounts of the current injected into the respective electrodes as described above, it is necessary to monitor not only the output of the SLD exiting light, but also the spectral form, so that the detector 811 is required to have a function as a spectrum analyzer, and at the same time it takes a time to exactly reset all current amounts injected into the plural electrodes when driving conditions deviate from initially set values due to some cause.
In addition, when a method of monitoring SLD light exiting from a face opposite to a face to use the SLD light (corresponding to FIG. 8B) is supposed, the following problem is caused.
Since carrier densities of respective electrodes are different when plural electrodes are used, the spectrum of stimulated and emitted light also has a distribution in a waveguide direction. Accordingly, since the same optical output and spectral form as the SLD light used are not obtained from the opposite exit face, exact feedback cannot be conducted even when the SLD light exiting from the opposite face is monitored.